This invention relates to apparatus for performing electrical resistivity surveying.
In some circumstances variations in the electrical conductivity of the subsurface can be mapped by making measurements of ground resistivity on the surface. Since rock types differ considerably in their conductivity, provided the structure is relatively simple, a picture of the subsurface geology can be built up from such measurements.
In a simple technique of resistivity surveying, four electrodes are driven into the ground to form an equi-spaced collinear array. Very low frequency alternating current is supplied to two of the electrodes and the potential difference is measured across the other two electrodes. Where the ground is horizontally layered a single set of measurements with increasing electrode separation is all that is necessary to carry out a quantitative interpretation. Where the boundary between different layers varies in one lateral direction, the data required for performing a quantitative interpretation of the subsurface conductivity can be obtained by traversing the four electrode array repeatedly over the same profile with progressively increased electrode separation. The maximum separation depends upon the maximum depth to which information is required. So far this has been done by using four separate cables and by moving the electrodes between readings. Where depths of 200-300 meters are being investigated over a profile of many miles, the labour required to perform the readings makes this method impracticable.